Exhaust Gas Boilers And Economisers for Marine Machinery Spaces

Exhaust Gas Boilers And Economisers for Marine Machinery Spaces

The use of exhaust gases from diesel main propulsion engines to generate steam is a means of heat energy recovery and improved plant efficiency.

An exhaust gas heat exchanger is shown in Figure above. It is simply a row of tube banks circulated by feedwater over which the exhaust gases flow. Individual banks may be arranged to provide feed heating, steam generation and superheating. A boiler drum is required for steam generation and separation to take place and use is usually made of the drum of an auxiliary boiler.

Auxiliary steam plant system

The auxiliary steam installation provided in modern diesel powered tankers usually uses an exhaust gas heat exchanger at the base of the funnel and one or perhaps two watertube boilers .

Saturated or superheated steam may be obtained from the auxiliary boiler. At sea it acts as a steam receiver for the exhaust-gas heat exchanger, which is circulated through it. In port it is oil-fired in the usual way.


Fig: Auxiliary steam plant system

Fig: Auxiliary steam plant system

Exhaust Gas Boilers And Economisers Arrangement

Auxiliary boilers on diesel main propulsion ships, other than tankers, are usually of composite form, enabling steam generation using oil firing or the exhaust gases from the diesel engine. With this arrangement the boiler acts as the heat exchanger and raises steam in its own drum.


Sketch of a steam economiser

Sketch of a steam economiser

General guideline

It is the responsibility of the Chief Engineer to ensure that all members of the Engine room Staff are familiar with the procedures for the safe operation and maintenance of the exhaust gas boiler/economisers.

Principles of safe operation, such as ensuring that valves and mountings are operational and correctly set, and the necessity to warm through and cool down in a gradual manner are similar to those detailed previously for oil fired boilers. As before, the manufacturers recommendations concerning operations must always be followed.


Soot fires in Boilers and Economisers

Soot fires in exhaust gas fired boilers and economisers are extremely dangerous, catastrophic and can seriously jeopardise the safety of the vessel. Ship’s staff should be aware of the dangers and methods of prevention.

They are caused by soot deposits from diesel engines running on heavy fuels generally when running at light load, or when manoeuvring where no exhaust gas bypass exists. These deposits may also contain excess cylinder lubricating oil and are highly susceptible to ignition once engine power and exhaust temperatures increase.

It is therefore imperative that tubes are maintained in a clean condition by the effective use of soot blowers. In particular, following periods of slow running or manoeuvring for extended periods, it is essential that soot blowing operations are carried out prior to an increase in power, or in the case of shutting down an engine. “Sootfires cannot occur in a heat exchanger which is free of soot deposits.”

It is also worth noting that the boiler water circulation pump is to be started well in advance of start up of the main engine, and not stopped for at least 2 to 3 hours after the main engine has been shut down, in order to ensure adequate cooling of the tubes.


Operating Instructions for Exhaust Gas Boiler/Economiser

These operating advices are not intended to be a comprehensive procedure, but merely an aide-memoire. The actual manufacturer’s instructions coupled with practical on-board experience will determine the actual sequence to be followed. i) Start up from cold

1. Ensure drain valves are shut.
2. Open air vent.
3. Ensure circulating pump valves are open.
4. Crack open inlet valve and close air vents when steam/water issues.
5. Slowly open inlet/outlet valve.
6. Check valves, flanges etc. for leakage.
7. Allow unit to warm through.
8. Start circulating pump and ensure stand-by unit on auto mode.
9. Check pump pressures, motor load etc.
10. Check that the gas side differential pressure manometer is operational.

ii) Start up from Hot If the system has been temporarily stopped, ensure that full circulation of water is established before gas is allowed through boiler. It is the responsibility of the Chief Engineer, or the Second Engineer in his absence, to ensure that a Circulating Pump is running and operating correctly before starting the Main Engine.

In the event that the main engine has been operating, with exhaust temperatures above 200 degree C for any length of time, without the circulating pump running, the engine must be slowed down and exhaust temperatures in and out of the exhaust gas boiler to be brought to below 100 Centigrade before starting a circulating pump. The consequences of starting a circulating pump with the exhaust gas boiler tubes at elevated temperatures could be disastrous.

On vessels with an exhaust economiser by pass valve, water temperature is a critical factor in preventing condensation and acid formation on the gas side. The minimum temperature of 140 degree C should be kept, when running with the exhaust gas bypassing the economiser.

iii) Operation 1) Exhaust gas boiler circulation must be maintained at all times. This is to maintain tube temperature and prevent “cold end” corrosion of tube and fins.

2) Soot blowing to be carried out at least three times daily.

3) If it is necessary to stop circulation for any reason, the unit should be soot-blown prior to shut down.

4) It is essential to take care that when slow steaming or running the main engine in a condition where the exhaust produced is likely to foul the gas side surfaces (auxiliary blower running), if fitted the gas by-pass should be used.

5) Monitor the differential pressure across the gas side and high exhaust gas outlet temperature alarm if fitted. 6) If soot release chemical is available then inject as per the manufacturers instructions.

CAUTION: When main engine speed is increased after a period of slow steaming, sootblowing should be increased and engine power increased very slowly whilst monitoring the economiser parameters.

iv) Normal Shutdown

1. Immediately prior to the end of sea passage operate the sootblowers.
2. Maintain circulation for a minimum of 12 hours after engine shutdown.
3. Shut down circulating pumps.

NOTE: Vessels with short periods of Port time such as Car carrier and Container vessels should not stop the circulating pump either at anchorage or in port.

4. Shut boiler isolating valves.
5. Open drains and vent valve.
6. During the port stay monitor the economiser unit casing for signs of high temperature.

v) Emergency Shutdown (Soot fire or tube failure)

1. Operate sootblowers.
2. Stop the circulation pump and isolate the unit.
3. Open the drain valves.
4. Depending upon circumstances stop the main engine.
5. Monitor the tube bank for soot fire progression.

CAUTION: If the tubes are intact, circulation can be continued and the stand by pump also started to improve circulation but the feed supply must be closely monitored. There is danger of a hydrogen fire should a tube fail; also water entering the main engine could cause severe damage to pistons and connecting rods due to a “hydraulic lock”.


Exhaust gas economiser efficiency

The Exhaust gas economiser in a ship is like a huge heat exchange that exchanges heat between exhaust gas from engines to water and produces steam for the same purpose that auxiliary boilers produce steam.

The revered energy and amount of steam generate by exhaust gas economiser is normally sufficient for routine ship-board steam requirements, thus normally a ship with exhaust gas economiser does not need to fire the boiler while in passage. As far as improving efficiency by avoiding the fouling on the gas side and water side, the same principles discussed under boiler applies.

The efficiency of an exhaust gas economiser can be improved by increased soot blowing frequency (once or twice a day while at sea). Recording the exhaust gas temperature difference and pressure drop can provide an indication of economiser cleanliness. Water washing should be scheduled into major repair periods.

The exhaust gas economiser maintenance will not only improve energy efficiency but also reduce maintenance overall costs and reduce safety risks associated with soot fires. Occasionally use of fuel additives may improve the cleanliness of the economiser .

As for ship design, the maximum waste heat recovery is desirable. For exhaust gas economisers, the funnel stack temperature must be as low as possible but with sufficient margin to be above the dew point to avoid sulphur corrosion. Generally a funnel temperature of 165 to 195 deg C when using fuel oil is considered optimum.

Boiler Cleanliness

i) Water Washing Sootblowing will remove dry deposits from the tube banks but for heavier accumulations water washing is necessary. Experience and inspection will determine the frequency necessary however, the normal requirement is to carry out water washing every 500 hours of operation or once a month.

1. Shut down boiler and allow unit to cool.
2. Water wash using a hand held hose or lance. Salt water may be used for initial washing although fresh water is preferable, usual quantity in the region of 15 – 20 tons. To aid the removal of acidic soot deposits, a mildly neutralising detergent may be added to the water wash.

3. If fouling is severe and deposits are difficult to remove, it may be necessary to apply chemical detergent solutions directly to the tube surfaces prior to washing. Allow time to soak before hosing down.

4. Final rinsing of tubes should be at least 30 minutes with fresh water to remove all salt water if used and/or cleaning chemicals.

CAUTION: Prior to commencement of water washing ensure all drains etc. are clear and that precautions have been taken to avoid water entering turbochargers and the main engine.

Upon completion of water washing, it is essential that the economiser tube bank be closely examined for any remaining soot deposits. Close up examination from within the economiser casing is required.

CAUTION: The possibility of soot fire increases after washing due to any remaining cold wet soot deposits, and also sparks being emitted from the funnel. Precautions are to be taken, mooring ropes protected etc, and fire-fighting equipment is to be in a state of readiness.

ii) Soot Collection

The majority of vessels are fitted with a soot collection tank for soot/wash retention while water washing exhaust Gas Boilers or Economisers.

Some designs have the soot discharged directly overboard via an educator driven by the fire main or similar arrangement. Others being of simple collection tank and “weir system” whereby the soot is retained within the tank with wash water returning to bilge for discharge overboard via the OWS. The approved plans for the vessel’s arrangements should be onboard.

With regard to the “weir system” the soot is then collected and retained onboard for disposal ashore. Whenever soot is landed ashore for disposal, pertinent details are to be recorded in the Garbage Log Book .


Dry Running

The practice of operating an exhaust gas boiler without circulation through the tubes should be avoided except in emergency situations. The following normal dry running procedures must be adhered to:

• Inform respective Management Office regarding the situation.
• Read instruction manual section before operating the equipment in a dry condition.
• The engine exhaust gas temperature must be no higher than the expected maximum.
• The period of dry running must be as short as possible.
• Prior to dry running the tube banks must be cleaned to remove all traces of soot and other accumulations.
• Cleaning by sootblowing must continue throughout the dry running period with at least the same frequency as for normal operation.
• The tube bank must be drained and vented.
• Monitoring of inlet and outlet gas temperatures is important to ensure no sudden rise occurs. Temperature excursions may indicate the presence of a sootfire.

Why is the gas outlet temperature kept about 180°C in an exhaust gas heat exchanger? - The gas temperature outlet in an exhaust gas heat exchanger is kept above 180°C to prevent low temperature corrosion occurring. There is about 10% water vapour in exhaust gas and also sulphur products and this can lead to suphuric acid forming which has a dew point of 140°C. The amount of exhaust gas energy used in waste heat recovering systems is about 5 ­ 10 %.


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